Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-Carrier Frequency Division Multiple Access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE (Release 8).
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE (Release 8) each sub-frame is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols are thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a sub-frame, assuming that the user allocation can change from sub-frame to sub-frame. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI. The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories.
Shared Control Information (SCI) carrying Cat 1 information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Down/ink Control Information (DCI) carrying Cat 2/3 information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence number        
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE Release 8/9 (3GPP LTE) are described in detail in 3GPP TS 36.212, “Multiplexing and channel coding (Release 9)”, version 8.8.0 or 9.0.0, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference).
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.        Hybrid ARQ (HARQ) information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).                        UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier. This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.        The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different flavors how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e. is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RVO. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Physical Downlink Control Channel (PDCCH)
The physical downlink control channel (PDCCH) carries the L1/L2 control signaling, i.e. transmit power control commands and the scheduling grants for allocating resources for downlink or uplink data transmission. To be more precise, the downlink control channel information (i.e. the DCI contents, respectively, the L1/L2 control signaling information) is mapped to its corresponding physical channel, the PDCCH. This “mapping” includes the determination of a CRC attachment for the downlink control channel information, which is a CRC calculated on the downlink control channel information being masked with an RNTI, as will explained below in more detail. The downlink control channel information and its CRC attachment are then transmitted on the PDCCH (see 3GPP TS 36.212, sections 4.2 and 5.3.3).
Each scheduling grant is defined based on Control Channel Elements (CCEs). Each CCE corresponds to a set of Resource Elements (REs). In 3GPP LTE, one CCE consists of 9 Resource Element Groups (REGs), where one REG consists of four REs.
The PDCCH is transmitted on the first one to three OFDM symbols within a sub-frame. For a downlink grant on the physical downlink shared channel (PDSCH), the PDCCH assigns a PDSCH resource for (user) data within the same sub-frame. The PDCCH control channel region within a sub-frame consists of a set of CCE where the total number of CCEs in the control region of sub-frame is distributed throughout time and frequency control resource. Multiple CCEs can be combined to effectively reduce the coding rate of the control channel. CCEs are combined in a predetermined manner using a tree structure to achieve different coding rate.
In 3GPP LTE (Release 8/9), a PDCCH can aggregate 1, 2, 4 or 8 CCEs. The number of CCEs available for control channel assignment is a function of several factors, including carrier bandwidth, number of transmit antennas, number of OFDM symbols used for control and the CCE size, etc. Multiple PDCCHs can be transmitted in a sub-frame.
Downlink control channel information in form of DCI transports downlink or uplink scheduling information, requests for aperiodic CQI reports, or uplink power control commands for one RNTI (Radio Network Terminal Identifier). The RNTI is a unique identifier commonly used in 3GPP systems like 3GPP LTE (Release 8/9) for destining data or information to a specific user equipment. The RNTI is implicitly included in the PDCCH by masking a CRC calculated on the DCI with the RNTI—the result of this operation is the CRC attachment mentioned above. On the user equipment side, if decoding of the payload size of data is successful, the user equipment detects the DCI to be destined to the user equipment by checking whether the CRC on the decoded payload data using the “unmasked” CRC (i.e. after removing the masking using the RNTI) is successful. The masking of the CRC code is for example performed by scrambling the CRC with the RNTI.
In 3GPP LTE (Release 8) the following different DCI formats are defined:                Uplink DCI formats:                    Format 0 used for transmission of UL SCH assignments            Format 3 is used for transmission of TPC commands for PUCCH and PUSCH with 2 bit power adjustments (multiple UEs are addressed)            Format 3A is used for transmission of TPC commands for PUCCH and PUSCH with single bit power adjustments (multiple UEs are addressed)                        Downlink DCI formats:                    Format 1 used for transmission of DL SCH assignments for SIMO operation            Format 1A used for compact transmission of DL SCH assignments for SIMO operation            Format 1B used to support closed loop single rank transmission with possibly contiguous resource allocation            Format 1C is for downlink transmission of paging, RACH response and dynamic BCCH scheduling            Format 1D is used for compact scheduling of one PDSCH codeword with precoding and power offset information            Format 2 is used for transmission of DL-SCH assignments for closed-loop MIMO operation            Format 2A is used for transmission of DL-SCH assignments for open-loop MIMO operation                        
For further information on the LTE physical channel structure in downlink and the PDSCH and PDCCH format, see Stefania Sesia et al., “LTE—The UMTS Long Term Evolution”, Wiley & Sons Ltd., ISBN 978-0-47069716-0, April 2009, sections 6 and 9.
Blind Decoding of PDCCHs at the User Equipment
In 3GPP LTE (Release 8/9), the user equipment attempts to detect the DCI within the PDCCH using so-called “blind decoding” (sometimes also referred to as “blind detection”). This means that there is no associated control signaling that would indicate the CCE aggregation size or modulation and coding scheme for the PDCCHs signaled in the downlink, but the user equipment tests for all possible combinations of CCE aggregation sizes and modulation and coding schemes, and confirms that successful decoding of a PDCCH based on the RNTI. To further limit complexity a common and dedicated search space in the control signaling region of the LTE component carrier is defined in which the user equipment searches for PDCCHs.
In 3GPP LTE (Release 8/9) the PDCCH payload size is detected in one blind decoding attempt. The user equipment attempts to decode two different payload sizes for any configured transmission mode, as highlighted in Table 1 below. Table 1 shows that payload size X of DCI formats 0, 1A, 3, and 3A is identical irrespective of the transmission mode configuration. The payload size of the other DCI format depends on the transmission mode.
TABLE 1DCI Formatspayload sizetransmissionpayload size Xdifferent from Xmode0/1A/3/3A1Cbroadcast/unicast/paging/power control1Mode 1DL TX modes1Mode 22AMode 32Mode 41BMode 51DMode 61Mode 71Mode 1SPS-Modes1Mode 22AMode 32Mode 41Mode 7
Accordingly, the user equipment can check in a first blind decoding attempt the payload size of the DCI. Furthermore, the user equipment is further configured to only search for a given subset of the DCI formats in order to avoid too high processing demands.
Medium Access Layer (MAC)
The MAC layer is one of the sub-layers of the Layer 2 in the 3GPP LTE radio protocol stack. The MAC layer performs (de)multiplexing between logical channels and transport channels by (de)constructing MAC PDUs (Protocol Data Units), also known as transport blocks. MAC PDUs are constructed out of MAC SDUs (Service Data Units) received through one or more logical channels in the transmitter. On the receiver side the MAC PDUs are reconstructed out of the received MAC PDUs.
The transport block (MAC PDU) consists of a header and a payload. Apart from MAC SDUs the payload can consist of MAC Control Elements and padding.
MAC Control Elements
For peer to peer signaling on MAC level MAC Control Elements (CEs) are used. MAC Control Elements can be part of a MAC PDU's payload as described above and are identified by a specific Logical Channel ID (LCID) in the MAC header.
There are several types of MAC CEs. Some of them are only included in uplink transport blocks for signaling from user equipment to eNodeB, others only in downlink transport blocks for signaling from eNodeB to user equipment. The special LCIDs and the corresponding MAC Control Elements transmitted on the downlink are listed in Table 2.
TABLE 2LCID valueMAC Control Element used for11100UE Contention Resolution Identity11101Timing Advance Command11110DRX Command
The special LCIDs and the corresponding MAC Control Elements transmitted on the uplink are listed in Table 3.
TABLE 3LCID valueMAC Control Element used for11010Power Headroom Report11011C-RNTI11100Truncated Buffer Status Report (BSR)11101Short BSR11110Long BSRSounding Reference Signals (SRS)
Sounding reference signals are send in the uplink. Together with the Demodulation Reference Signals (DM RS) they are included in the uplink to enable channel estimation for coherent demodulation as well as channel quality estimation for uplink scheduling.
While DM RSs are associated with the transmission of uplink data, the SRSs are not associated with data transmission and primarily used for channel quality estimation to enable frequency-selective scheduling by the scheduling eNodeB. Furthermore SRSs can be used to enhance power control or to support the eNodeB in deciding on initial Modulation and Coding Scheme (MCS) for data transmission. If configured by higher layer signaling, the SRSs are transmitted in the last SC-FDMA symbol in a uplink sub-frame. The sub-frame in which SRSs are to be transmitted by the user equipment is indicated by cell-specific broadcast signaling and is selected out of a set of 15 possible sub-frames within a radio frame. Data transmission on the Physical Uplink Shared CHannel (PUSCH) is not allowed in the sub-frame designated for transmitting SRSs, which sets the SRS overhead to 7% when all possible sub-frames are configured for SRS transmission. As mentioned above, SRS configuration is done by the eNodeB using higher layer signaling. The configuration inter alia determines amongst other parameters duration and periodicity of the SRSs.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved in the 3GPP. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
In Carrier Aggregation (CA), two or more Component Carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100 MHz. All component carriers can be configured to be 3GPP LTE (Release 8/9) compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. This does not necessarily mean that all component carriers need to be compatible to 3GPP LTE (Release 8/9).
A user equipment may simultaneously receive or transmit on one or multiple component carriers. On how many component carriers simultaneous reception/transmission is possible, is depending on the capabilities of a user equipment.
A 3GPP LTE (Release 8/9) compatible user equipment can receive and transmit on a single CC only, provided that the structure of the CC follows the 3GPP LTE (Release 8/9) specifications, while a 3GPP LTE-A (Release 10) compatible user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the user equipment only has one Radio Resource Control (RRC) connection with the network. One cell—the “special cell”—provides the security input and the Non-Access Stratum (NAS) mobility information (e.g. TAI). There is only one special cell per user equipment in connected mode.
After RRC connection establishment to the special cell, the reconfiguration, addition and removal of component carriers can be performed by RRC. At intra-LTE handover, RRC can also add, remove, or reconfigure component carriers for usage in the target cell.
When adding a new component carrier, dedicated RRC signaling is used for sending component carriers' system information which is necessary for component carrier transmission/reception, similar to a handover in 3GPP LTE (Release 8/9).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always activate. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling.
The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier. FIG. 7 and FIG. 8 exemplarily show possible linkages between downlink and uplink component carriers. While in FIG. 7 all downlink component carriers are linked to the same uplink component carrier, in FIG. 8 downlink component carriers 1 and 2 are linked to uplink component carrier 1 and downlink component carrier 3 is linked to uplink component carrier 2.
DRX and Carrier Aggregation
In order to provide reasonable battery consumption of user equipment 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX).
For this concept the following terms describe the user equipment's state in terms of DRX.                on-duration: duration in downlink sub-frames that the user equipment waits for, after waking up from DRX, to receive PDCCHs. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer;        inactivity-timer: duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH, failing which it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions).        active-time: total duration that the user equipment is awake. This includes the “on-duration” of the DRX cycle, the time user equipment is performing continuous reception while the inactivity timer has not expired and the time user equipment is performing continuous reception while waiting for a downlink retransmission after one HARQ RTT (Round Trip Time). Based on the above the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite);        
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern.
In order to allow for further battery saving optimization, a further step of activation/deactivation of component carriers is introduced. Essentially a downlink component carrier could be in one of the following three states: non-configured, configured but deactivated and active. When a downlink component carrier is configured but deactivated, the user equipment does not need to receive the corresponding PDCCH or PDSCH, nor is it required to perform CQI measurements. Conversely, when a downlink component carrier is active, the user equipment shall receive PDSCH and PDCCH (if present), and is expected to be able to perform CQI measurements. After configuration of component carriers in order to have PDCCH and PDSCH reception on a downlink component as described above, the downlink component carrier needs to be transitioned from configured but deactivated to active state.
In the uplink however, a user equipment is always required to be able to transmit on PUSCH on any configured uplink component carrier when scheduled on the corresponding PDCCH (i.e. there is no explicit activation of uplink component carriers).
For user equipment power-saving purposes, it's crucial that additional component carriers can be de-activated and activated in an efficient and fast way. With bursty data-transmission, it is imperative that additional component carriers can be activated and de-activated quickly, such that both the gains of high bit-rates can be utilized, and battery preservation can be supported. As described before user equipments will not perform and report CQI measurements on configured but deactivated downlink component carriers but only radio resource management related measurements like RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality) measurements. Hence when activating a downlink component carrier, it's important that eNodeB acquires quickly CQI information for the newly activated component carrier(s) in order to being able to select an appropriate MCS for efficient downlink scheduling. Without CQI information eNodeB doesn't have knowledge about user equipment's downlink channel state and might only select a rather conservative MCS for downlink data transmission which would in turn lead to some resource utilization inefficiency.
In order to acquire CQI information quickly, eNodeB can schedule an aperiodic CQI by means of an uplink scheduling grant. The aperiodic CQI would be transmitted on the physical uplink shared channel (PUSCH). Therefore in order to activate a configured downlink component carrier, eNodeB would need to issue essentially two grants (PDCCH) to the UE, one downlink PDCCH in order to indicate the activation of a downlink component carrier and one uplink PDCCH which schedules uplink resources for the transmission of the aperiodic CQI. Furthermore both PDCCH has to be sent respectively received in the same TTI in order to ensure, that user equipment measures and reports CQI information for the correct downlink component carrier, i.e. the downlink component carrier which will be activated.
The correct reception of the aperiodic CQI can serve as an acknowledgement for the downlink activation command, i.e. when aperiodic CQI has been received eNodeB assumes that user equipment has activated the downlink component carrier indicated in the downlink PDCCH.
As it becomes apparent, the main drawback of the above described component carrier activation method is, that two PDCCHs are required in order to activate a downlink component carrier. Furthermore due to the fact that the two PDCCHs need to be received/sent simultaneously, certain error cases may occur in the presence of PDCCH loss.
In case only the downlink “activation” PDCCH is lost, user equipment will not activate the downlink component carrier. However based on received CQI information eNB erroneously assumes downlink activation has succeeded.
In the second error case when only the uplink PDCCH which requests the aperiodic CQI is lost, eNodeB doesn't acquire CQI and erroneously assumes that downlink activation has failed.